Spring contact probes are typically used for testing electronic components themselves or the components mounted on printed circuit boards (PCBs), flex circuits or substrates. The probes, preferably being retractable, are also used to test connectors, back planes and other circuitry where a number of points must be accessed. Referring to FIG. 1, there is shown a spring contact probe 10 having a contact head 12, a plunger 14, a housing or probe body 16, a plunger body 18, and a spring 22. Optionally, the probe 10 includes a crimp 20 on the housing to restrict the movement of the plunger 14 and plunger body 18. The spring 22 is preferably wedged in the closed area of the probe housing which optionally has a vent hole 24 at the bottom. This probe is sufficient for testing electronic products in low frequency applications when the surface being contacted is substantially flat and the test does not require a high degree of accuracy.
The spring contact probe 10 presents a problem when the contacting surface is curved and the test requires a high degree of accuracy. The introduction of Controlled Collapsible Chip Carrier Connections (C-5) on substrates illustrates the incompatibility of the spring contact probe 10 with the use of present contacting schemes. C-5 is a contacting scheme using solder spheres on a component that are reflowed onto the contact side of a surface mount package such as a chip carrier. The present design of spring contact probes as shown in FIG. 1 fails to conform to the spherical contour of the C-5 ball. A minor contact misalignment between the solder ball and the spring contact probe will cause a single load-bearing spot, which in turn causes an area of constriction resistance. The misalignment changes the coaxial geometry of a radio frequency (RF) transmission line and causes an increase in the characteristic impedance of the line. The impedance change due to the misalignment is uncontrolled and unpredictable and obviously degrades the accuracy and repeatability of RF tests and measurements. Thus, a need exists for a spring contact probe that will compensate for the misalignment of contact surfaces of electronic components in order to obtain better contact surfaces and more accurate and repeatable RF tests and measurements.